I love the concept of this. Not so much to capture energy from individuals who are using the devices purely to produce energy, but to capture the energy produced from large groups doing something that they would be doing anyway, e.g. people using exercise bikes and treadmills at a gym, or prisoners exercising in a prison exercise yard.

But, unfortunately, some quick sums suggest this might not be particularly economically viable:

- 75W sustainable energy output per person, i.e. 0.075kWh

- 33% efficiency of energy conversion, i.e. 0.025kWh

- GBP0.05 per kWh to sell back to grid, i.e. GBP0.00125 per person per hour

- 8 hour day = GBP0.01 (i.e. 1p) per person per day

- 100 people in a gym, or prison, or whatever = GBP1 per group per day

i.e. unlikely to cover the cost of lighting and heating. Even if you could improve to 100% efficiency, and offset against electricity purchased rather than reselling back to the grid, you'd still only be looking at increasing that by a factor of 10.